WO 2009/144534 A1 relates to measurement of a transponder device's (PD or Proximate Device) response time at a reader device (PCD or Proximity Coupling Device) and describes that proximity of the transponder device (i.e. that the transponder device is in proximity of the reader device) may be checked by comparing the measured transponder device response time with a predetermined threshold. Thereby, the risk of relay attacks may be reduced, i.e. attacks where another transponder device and another reader device that are both controlled by the attacker(s) are interposed between legitimate reader and transponder devices.
However, the above approach cannot prevent relay attacks in cases where the reader needs to manage a population of transponder devices with different response times (e.g. due to different implementations). For example, if the maximum response time of a valid transponder device implementation is 1 second and another valid transponder device implementation provides a valid transponder response after 750 ms, a 250 ms window will be available for relay. Such an early transponder response may e.g. be provided by different implementations (e.g. standalone vs. mobile).
Accordingly, there may be a need for an improved way of checking whether a transponder device is in proximity of a reader device, in particular an improvement that is capable of handling the above cases and which is easy to implement.